The invention relates to a piston engine comprising at least one piston which is movable in a reciprocating manner in a gas-filled cylinder and divides the cylinder into a first chamber and a second chamber, while during operation a gap is provided between an outer wall of the piston and an inner wall of the cylinder, which gap communicates the first chamber with the second chamber and is different in size in annular cross-section transverse to the direction of movement of the piston.
In a known (see Proceedings 12th International Cryogenic Engineering Conference, Southampton, United Kingdom, July 1988) piston engine of the kind mentioned in the opening paragraph, it is found that both with an annular circular-cylindrical gap and with an annular wedge-shaped gap between piston and cylinder a change in the central position of the piston occurs due to an inequality between the mass flows (kg per machine cycle) of the gas moving in one direction through the gap and of the gas moving in the opposite direction through the gap. The said inequality results in that the piston is subjected to a force which is oppositely directed to the direction of the larger mass flow of the relevant mass flows. Thus, in order to obtain with a reciprocating piston not mechanically coupled to a drive (a so-called free piston) a constant and stable central position, the piston must be subjected to a compensation force which is related to the force due to the difference in mass flows. In general, this leads to a comparatively expensive piston engine, the more so as the force caused by the displacement and the instability of the central position is not constant, or fixed, under practical conditions.